This invention relates to the use of optics to produce a directed beam of radiation by grazing-incidence reflection. More particularly, an optical element is disclosed which is produced by assembling a collection of separate tapered-monocapillary optical elements to form a polycapillary optic. The individual monocapillary channels are created in a batch process which allows for the optimization of the shape, smoothness, and material choice of the radiation-reflecting interior surfaces. The resultant optics can be used to produce either a collimated beam or a focused spot of photons, neutrons, or charged particles. In the case of photons, the use of x-rays is the most important application.
In recent years, there have been dramatic developments in the field of x-ray optics. Many different types of optical elements have been introduced or improved for the manipulation of short wavelength photons. In the specific area of focusing optics, several different types of lenses and focusing mirrors have been produced. These optics. include: zone plates, Bragg-Fresnel zone plates, multilayer focusing mirrors, grazing incidence mirrors, compound refractive lenses, and capillary optics. In addition to photons, some of these optical elements can be used for the focusing of neutrons and charged particle beams.
The introduction of x-ray analysis has been one of the most significant developments in twentieth-century science and technology. The use of x-ray diffraction, spectroscopy, imaging, and other techniques has led to a profound increase in our knowledge in virtually all scientific fields. The capabilities of x-ray analysis have expanded consistently with the availability of ever more powerful sources of radiation. The standard x-ray tube has seen a relatively gradual increase in performance over many decades. Notable improvements in x-ray tube technology include the introduction of rotating anode sources and microfocus tubes. The advent of synchrotron radiation sources over the past few decades has led to a true revolution in x-ray science. Although the use of synchrotron radiation has become an extremely important research tool, the need to travel to large and extremely expensive central facilities to perform experiments during a limited time interval is a distinct disadvantage. Thus, the vast majority of work is still performed using x-ray tubes.
Many experiments are now performed using rotating anode sources which have significantly higher power capabilities than stationary anode tubes. These sources are quite expensive and can consume over ten kilowatts of input power. Recently, with the introduction of improved x-ray focusing optics, the ability to use small, low power microfocus x-ray sources to achieve x-ray beam intensities comparable to that achieved with rotating anode tubes has been demonstrated. It has been shown that a microfocus source running at a few tens of watts input-power, in conjunction with focusing optics, can produce beams with a brightness comparable to a multi-kilowatt rotating anode source. Such combined small sources and collection optics will greatly expand the capabilities of x-ray analysis equipment in small laboratories. The optimization of x-ray optics for these applications is of crucial importance for realizing the potential of these laboratory instruments.
In addition to sources of x-rays, there has been a significant increase in the capabilities of neutron sources, many of which exist as user facilities in a manner similar to synchrotron radiation sources. Both reactor and spallation sources have been built with ever increasing neutron fluxes. For certain applications such as prompt-gamma activation analysis and neutron depth profiling, it would be very desirable to produce a focused beam of thermal or cold neutrons. The use of small beams is also advantageous for neutron diffraction applications, although the increased divergence of the focused beam can be detrimental in some cases. There have been some advances in neutron focusing optics over the past few years. Improvements to these optics will have a large impact on the capabilities of these neutron facilities.
Although there exist x-ray optics which utilize diffraction and refraction for their operation, we are concerned with reflective optics in this invention. It is well known that x-rays incident on a surface at sufficiently small angles of incidence will be reflected by total external reflection. The largest angle of incidence for reflection (critical angle) is determined by the refractive index of the material:
n=1xe2x88x92xcex4xe2x88x92ixcex2
Using Snell""s Law, we can derive this angle as:
xcex8c=(2xcex4)xc2xd(assuming xcex2=0)
The theoretical value for xcex4 is:
xcex4=xc2xd(e2/mc2)(N0xcfx81/A)Zxcex2=2.70xc3x971010(Z/A)xcfx81xcex2
The angles are quite small since the refractive index for x-rays is very close to unity for all materials. For example, the critical angle for borosilicate glass atxcex=1xc3x85 is less than 3 milliradians. For achieving the highest critical angles, high density materials such as gold or platinum are desirable.
In the case of neutrons, grazing incidence reflection can also be used for producing optical devices. The critical angle for reflection of neutrons is:
xcex8c=xcex(Nb/xcfx80)xc2xd
Where b is the coherent scattering length and N is the number of nuclei per cm3. The best natural material for achieving the highest critical angle for neutron reflection is nickel. The isotope Ni-58 is especially good, having a critical angle of approximately 2.1 milliradians/xc3x85.
In addition to single-layer reflecting materials, multilayer coatings can be produced which rely on Bragg reflection to achieve high reflectivity. These layers are composed of a high-Z material which exhibits large coherent scattering for the radiation being reflected, with an alternating low-Z material that functions as a spacer. In the case of x-rays, the most common high-Z materials are tungsten or molybdenum, while the low-Z spacer is usually silicon, carbon, or beryllium. In the case of neutron reflection, these multilayer coatings are often referred to as xe2x80x9csupermirrors.xe2x80x9d Neutron Supermirrors differ from standard x-ray multilayers in that the d-spacing of the layers is not constant, but increases for the layers towards the surface of the mirror. Supermirror structures are generally composed of layers of nickel or a nickel alloy, with spacer layers of titanium.
Reflective x-ray optics can be classified into several different categories. One class of optics uses grazing reflection from extended mirror surfaces. The most common mirror arrangements have an ellipsoidal or toroidal surface figure for two dimensional focusing of radiation. Another common geometry uses two spherical mirrors oriented sequentially in the vertical and horizontal planes, in an arrangement known as a Kirkpatrick-Baez configuration. An absolute requirement for all reflective x-ray optics is the need to have exceedingly smooth reflecting surfaces due to the small wavelength of the radiation. In general, the surface roughness should be better than 1 nanometer rms.
A different approach for reflective x-ray optics uses the ability of fine glass capillaries to act as reflective guide tubes for x-rays, in a similar manner to fiber optics. Several different configurations of these capillary optics exist. One type of optic, sometimes known as a xe2x80x9cKhomakhov Lensxe2x80x9d, uses a number of discreet curved glass-capillary tubes which are precisely mounted in a frame which independently holds each capillary""s curved position along the device. In some optics, each carefully positioned capillary fiber is actually a bundle of many much smaller capillary tubes. X-rays are guided through each capillary by multiple reflections along the outer arc of the capillary tube""s interior surface. This mode of reflection is sometimes referred to as a xe2x80x9cWhispering Galleryxe2x80x9d. With such optics, the divergent radiation from an x-ray tube""s focal spot can be redirected into either a collimated beam, or condensed back to a small spot a significant distance from the source. This type of optic is not capable of producing spots smaller than approximately 500 microns; the diameter of each fiber. This type of optic is sometimes referred to as a xe2x80x9cmultifiber optic.xe2x80x9d
A different type of multicapillary optic uses a single multicapillary bundle which is drawn at elevated temperature so as to have a taper on one or both ends. This type of optic is sometimes referred to as a xe2x80x9cmonolithic optic.xe2x80x9d Unlike the multifiber optic, the individual glass channels do not have a constant diameter along their length. This type of optic functions in a similar manner to the multifiber type of optic, but has certain properties which are advantageous in some applications. The smallest spot sizes achievable with this type of optic are approximately 20 microns or larger, while quasi-parallel beam sizes are generally several mm in diameter. Both types of multicapillary optics possess the very attractive ability to collect a large solid angle of radiation emitted from the source. Their main disadvantage is their relatively high cost, and their inability to form very small focused or collimated beams. Both the multifiber and the monolithic optics can also be used as bending optics by curving the capillary bundle. It is possible for a single optic to have a bending section and a focusing section.
Both the monolithic and the multifiber optics are manufactured and sold by X-ray Optical Systems which is located in Albany, N.Y. This company holds a number of patents regarding this technology.
In addition to the previously described capillary optics having multiple channels, a single tapered-capillary may be used effectively to collimate or focus radiation. This tapered monocapillary optic has achieved the smallest spot sizes of any type of x-ray optical device. Their most dramatic use has been with synchrotron radiation where the almost parallel input beam can be condensed to sizes well below 1 micron (≈0.05 microns has been achieved). It is worth noting that tapered monocapillary optics can function in two different focusing modes. In one case, the capillary acts as a true focusing lens, with each photon undergoing a single bounce. In this case, a focal spot is produced some distance beyond the capillary exit. In a different type of capillary optic, photons undergo multiple bounces on their way to the exit and the smallest beam diameter is found directly at the exit aperture. This type of optic is often referred to as a capillary condenser, to draw the distinction from a true lens. The smallest beams have been produced by the condenser type optics, although the small working distance from the exit aperture can be a disadvantage.
The monocapillary type of optic is usually produced by heating and drawing a length of glass capillary tubing to a smaller diameter. There has been some progress in forming glass capillaries with paraboloidal or ellipsoidal shapes, but the slope errors and straightness of the capillary shapes have been difficult to reliably reproduce. It has also not been feasible to coat the inside of these very small capillaries with a different material to enhance the reflectivity. In addition to the standard monocapillary optics produced by this method, a different method has been devised to produce metal reflective tubes by a replicating process on a removable mandrel. The optics produced by these replication methods fall into a category somewhere between standard reflecting optics and true capillary optics, due to their larger bore size. The smallest dimensions of their bores are generally near 0.5-1 mm.
U.S. Pat. No. 5,772,903 (1998) entitled TAPERED CAPILLARY OPTICS describes a different technique to produce tapered monocapillary optics which have significant advantages over the tapered glass capillaries. That patent describes a method to produce a capillary structure having a well controlled taper profile, a high degree of straightness, an extremely smooth internal reflecting surface, and wide latitude in the selection of materials for the internal bore. The extremely small dimensions (1 micron) achievable by the methods delineated in that patent are unique for non-glass capillaries.
This invention relates to the use of a collection of tapered monocapillary optical elements produced by methods described in U.S. Pat. No. 5,772,903, which are bundled together to produce a new type of monolithic polycapillary lens. In this patent, a process to produce many identical tapered monocapillary elements, as well as methods to assemble them accurately to produce the polycapillary structure are disclosed.
The advantage of this type of optic, in comparison to standard glass polycapillary optics will be elucidated from the descriptions that follow. These advantages are a consequence of the fabrication methods, and are the following:
1) It is possible to reproducibly generate almost any desired shape for the individual capillary channels on demand.
2) The process permits wide latitude in the selection of materials of the capillary bores for optimizing the reflectivity of the radiation being used. This can even include multilayer coatings.
3) A highly absorbing material can surround the reflecting layer which minimizes scattered-radiation background.
4) The manufacturing process does not rely on drawing the capillary materials at elevated temperatures which can be difficult to control, and often introduces deformation.
A plurality of glass or metal wires are precisely etched to form the desired shape of the individual channels of the final polycapillary optic. This shape may be conical, paraboloidal, ellipsoidal, or some other shape. This shape is created by carefully controlling the withdrawal speed of a group of wires from an etchant bath. In the case of complete ellipsoidal capillary channels, the etching operation is performed twice in opposite directions on adjacent wire segments. The etched wires undergo a subsequent operation to create an extremely smooth surface. This surface is coated with a layer of material which is selected to maximize the reflectivity of the radiation being used. This reflective surface may be a single layer of material, or a multilayer coating for optimizing the reflectivity in a narrower wavelength interval. The collection of individual wires is assembled into a close-packed multi-wire bundle, and the wires are bonded together in a manner which preserves the close-pack configuration, irrespective of the local wire diameter. The initial wires are then removed by either a chemical etching procedure or mechanical force. In the case of chemical etching, the bundle is generally segmented by cutting a series of etching slots. Prior to removing the wire, the capillary array is typically bonded to a support substrate. The result of the process is a bundle of precisely oriented radiation-reflecting hollow channels. The capillary optic is used for efficiently collecting and redirecting the radiation from a source of radiation which could be the anode of an x-ray tube, a plasma source, the fluorescent radiation from an electron microprobe, a synchrotron radiation source, a reactor or spallation source of neutrons, or some other source.